1. Field of Invention
This invention relates to a speed control device for regulating the operating speed of electric motor-driven apparatuses, particularly, but not limited to, power tools and hand tools.
2. Description of Prior Art
Quite often it is required that the operating speed of an apparatus or power tool be varied and regulated by electrical means.
Early electric motor speed control methods employed a plurality of windings, variable series-connected resistances, or variable voltage output transformers to control the operating speed of the apparatuses or tool. Variations on this theme include U.S. Pat. Nos. 3,596,161 and 3,678,357 both to Raymond and Swanke (1969 and 1970, respectively) which include the provision of multiple windings or winding taps and a means of selection by which combinations of windings or taps in series or parallel could be connected, thus varying the operating speed of the apparatus. The addition of a solid-state rectifier that could be switched into a series connection with the motor windings provided additional speed settings by selectively allowing only one polarity of the AC power source half cycles to provide power for the motor. These methods tend to be bulky and may require many wiring connections, a complex electrical switch, as well as requiring complex motor coil winding methods. These factors make them unacceptable for many types of tools, especially where small size is of importance such as in the case of typical power and hand tools. These two patents are directed to and particularly well suited for kitchen blenders. These particular patents are mentioned as background only, as they do not embody modulation of the incoming power source as a control means.
More advanced speed control methods provide for variable modulation of the incoming power source by controlling the conduction timing of a semiconductor control device or a multitude of such devices connected in series with the motor winding or windings. U.S. Pat. No. 3,209,228 to Gawron (1965) is an early example of an embodiment of this concept, employing a silicon thyristor, a voltage trigger device, and a resistor-capacitor timing network. By varying the resistor value, the conduction timing of the thyristor may be varied, thus controlling the average voltage applied to the motor of the apparatus. U.S. Pat. No. 3,327,196 to Sahrbacker (1967) incorporates the same concept, but includes a different voltage trigger device. U.S. Pat. No. 3,329,842 to Brown (1967), U.S. Pat. No. 3,447,057 to Brown et al (1969), and U.S. Pat. No. 3,484,632 to Opalenik (1969) also describe variations on the same basic concept, but specifically as applied to power tool control "trigger"-style switches. These patents all feature an analog (infinitely variable) means of operator input, typically comprising a variable position "trigger" button. None encompass any form of discreet power level input selections, nor any form of output load or speed compensation.
U.S. Pat. No. 3,422,330 to Swanke (1965) provided discreet power level input selections in the form of a plurality of mechanical switches that provided for varying combinations of resistances as applied to a basic resistor-capacitor timing method. This is another variation on U.S. Pat. No. 3,209,228 previously mentioned, and also seems to be primarily intended for application to kitchen blenders.
Typical speed control methods prior to the present have employed open-loop control techniques, providing no means of speed compensation for varying power source or output load conditions. As one example of the desirability of such a feature, in the case of a cutting or drilling apparatus, the cutting tool may become jammed in the workpiece if the power output from the tool is not rapidly adjusted in compensation for varying load conditions. Problems of this nature are particularly troublesome at lower operational speeds. The response time required to prevent such jams is generally much shorter than a human operator can reliably perform. Automatic closed-loop speed compensation is therefore desirable. U.S. Pat. No. 4,454,459 to Siegfried (1981) and U.S. Pat. No. 4,734,629 to Lessig, Wheeler, Bailey and Smith (1988) both describe closed-loop motor load compensations based upon sensing the motor's rotational speed. The second is a variation of the concept of the first, and neither one provides a means for sensing or limiting output power overload conditions. They are also analog-logic based, as with the basic thyristor-based patents previously mentioned, and utilize a relatively costly feedback means (a tachometer generator).
Few previous speed control methods have provided for discreet electronic incremental control of the motor speed. U.S. Pat. No. 3,641,410 to Vogelsberg (1972) describes a "touch" control method which varies the output power based upon the location of an operator's finger contact upon variable resistance elements, or in another embodiment, based upon the operator's touch of one or more of several discreet touch points. U.S. Pat. No. 4,536,688 to Roger (1985) describes a system for selecting from predetermined speeds by means of a rotary selection switch in association with a mechanical ratio changing method. U.S. Pat. No. 4,636,961 to Bauer (1984) describes a control with a memory device (ROM) containing preset values for controlling a motor in an electric power tool. Vogelsberg's method (U.S. Pat. No. 3,641,410) does not provide for safety isolation of the operator from the power source, and does not provide for reproducible output speed settings when the operator's touch pressure or position varies. It also does not allow operations without the operator maintaining contact with the control touch points. Roger's method (U.S. Pat. No. 4,536,688) is at least partially mechanically based, and accordingly does not address smaller hand tools or the vast majority of cutting tools in which changeable drive ratios are too costly, impractical or otherwise undesirable. It is further limited to a very few speed settings and does not provide an incremental speed selection method. Bauer's method (U.S. Pat. No. 4,636,961) implies selection of the preset values as "material types" and does not describe provisions for incremental control during operation, appearing to be intended for stationary power tools such as drill presses, and thus also failing to address the needs of smaller hand tools.
Nearly all previous speed control methods have utilized analog operator input technologies, primarily sliding wiper potentiometers in one form or another. Because potentiometers are mechanical devices having sliding contacts, it is unavoidable that the resistive element and the sliding contact operating upon it should experience wear due to friction. Such wear will cause drift of the circuit's operating parameters, eventually leading to erratic operation or outright failure of the control circuit. U.S. Pat. Nos. 3,641,410, 4,536,688, and 4,636,961 as previously mentioned, do provide for discreet ("digital") inputs for control of output power levels, but they do not provide for simple "up-down" incremental control with reproducible settings.
U.S. Pat. No. 3,887,856 to Chicchiello (1975) does describe a pushbutton "up-down" speed control method. Although providing this function, it is "infinitely" variable, providing neither reproducible speed settings, nor discreet incremental control. In addition, it is based upon a purely analog control method and is therefore subject to drift. No memory of previously selected settings is provided. There is no compensation provided for the motor load. Finally, it is specifically intended and suited for control of film drive motors, and not for power tools or for general apparatus usage.
Active displays have not normally been provided to indicate the currently selected control output settings. U.S. Pat. Nos. 4,536,688 and 4,636,961 as previously described do provide for displays, but the former is at least in part associated with a mechanical means of selecting the output speed (mechanical ratio selections), and the latter is more of a "material type selection" display, not necessarily a speed selection display. The latter is also based upon a few fixed values stored in a fixed memory device rather than an operator-selectable setting from within a larger range.
All the speed control control methods previously described suffer from one or more of the following deficiencies, as applicable to electrically motor-driven apparatus, tools, and hand tools:
1. No "memory" of the power or motor speed settings previously selected have typically been provided except for the adjustable mechanical limit stops provided on many trigger-style speed controls. Where provided, the mechanical limit stops are themselves subject to mechanical wear and can be inconvenient to adjust when performing work with the apparatus or tool. Presence of said preset mechanical limit stops are also not conducive to regulation of the apparatus output speed under varying input power or output load conditions.
2. No automatic safety mechanisms to protect against motor overspeed in the event of a shorting failure of the semiconductor control device or devices have been provided by any of the previously mentioned methods, allowing a potential safety hazard to exist for the operator of the apparatus. In applications where universal or brush-equipped DC motors are used, such failures may lead to sudden unexpected motor overspeeds and uncontrolled accelerations.
3. No mechanisms to limit the maximum output power or torque have been provided, thus failing to protect the motor from overload or stalling conditions. This also allows a potential safety hazard to exist for the operator of the apparatus.
4. Previous analog-based methods have required adjustments of the circuit and/or its component values at time of manufacture to compensate for different line frequencies and voltages provided in various parts of the world. In some instances, entirely different versions of the circuitry have been required to adapt to varying local power source conditions.
5. Previous analog methods typically require the trimming of circuit elements at time of manufacture under simulated or actual operating conditions. This is normally necessary in order to obtain consistent control output levels, and is due in large part to variations in circuit component values.
6. Previous methods typically have not provided a smoothly ramped turn-on function. That is to say that they either switched on immediately to a selected output power level, or have provided no positive means to limit the acceleration of the apparatus' motor from rest to the desired operating speed. Rapid, uncontrolled accelerations may cause input power surges, premature wear of the motor's brushes (if so equipped), and significant turn-on torques from the acceleration of the motor armature. Motor torques will be transmitted to the operator or to the framework of the apparatus. Turn-on torques are of particular concern in the case of larger or more powerful hand tools because they may cause the operator to momentarily lose physical control of the tool, thereby creating an operator safety hazard. In applications involving cutting tools such as routers and saws, said turn-on torques may also lead to undesired roughness or misplacement of cuts on the work piece, even where the tool is mounted securely to a stationary framework. The same concerns are applicable as regards to smoothly ramping between selected speeds.
7. Previous methods have typically not provided a means to re-enable or retrigger the semiconductor switching device or devices should their operation be interrupted due to momentary mechanical interruption of the motor's power circuit. Interruptions of this type are typically caused by brush bounce in brush-type motors. Where semiconductor thyristor switching devices are used, said interruptions may cause the devices to switch off prematurely, thereby contributing to erratic operation of the apparatus or tool.